read_csv in combination with index_col and usecols

Starting point:

http://pandas.pydata.org/pandas-docs/stable/io.html#index-columns-and-trailing-delimiters

If there is one more column of data than there are colum names, usecols exhibits some (at least for me) unintuitive behavior:

>>> data = 'a,b,c\n4,apple,bat,5.7\n8,orange,cow,10'
>>> pd.read_csv(StringIO(data))
        a    b     c
4   apple  bat   5.7
8  orange  cow  10.0
>>> pd.read_csv(StringIO(data), usecols=['a', 'b'])
   a       b
0  4   apple
1  8  orange
>>>

I was expecting it to be equal to

>>> pd.read_csv(StringIO(data))[['a', 'b']]
        a    b
4   apple  bat
8  orange  cow

I am not sure if my expectation is unfounded, though, and that this behavior is indeed intentional?

[gfyoung]

That’s not a bug. index_cols is relative to usecols. In this, you only have one column that you want to extract from the CSV, but you want two columns for the index.

[gfyoung]

Exactly. That’s part of what I was proposing you do in #9098.